1. Field of the Invention
The present invention relates to an apparatus for and method of determining the orientation and/or position of a structure that produces a diffraction pattern. In particular, the invention may be applied to an optical component that produces a diffraction pattern. Examples of applications of the invention include alignment of a sensor in a body for high-resolution photogrammetry, and alignment of optical elements or grating microstructures in an optical arrangement. In particular, the invention is applicable to the alignment of an optical component, such as a mirror or a detector, of a camera, and to the alignment of two camera bodies relative to one another to produce a stereoscopic camera pair.
2. Description of the Related Art
Human beings have two eyes which are placed side-by-side in the face with a lateral separation that varies from person to person with an average of around 65 mm. These eyes see the three-dimensional world from two slightly different points of view. When an object is close to an observer, the left eye sees the object from a different angle than does the right eye, and such objects appear shifted, relative to objects at a greater distance from the observer, when the views from each eye are compared. This shift is known as “parallax”. The parallax is dependent on the distance of an object from the observer, so that the more distance is the object, the smaller is the shift or parallax. This behaviour is known as “binocular vision” and enables a person to judge the distance to an object and thus to assess the size of the object when no other cues (such as motion, memory or perspective) exist to judge the distance to an object. This ability to judge the distance to an object is called “stereopsis”, which means “solid seeing”.
The concept of stereopsis has been combined with photography to produce a three-dimensional camera. A single camera can produce a two-dimensional image, and the principle of three-dimensional photography is that two cameras are used, one to obtain the left eye image and one to obtain the right eye image. The two cameras are set up at a separation that is similar to the separation between the left eye and the right eye of a human being, so as to mimic the stereopsis effects. Each photograph mimics the image obtained by one eye, so that the two images include the parallax and shifts that are needed to judge distance by stereopsis. The two images are displayed to an observer such that the left-eye image is displayed to the observer's left eye and the right eye image is displayed to the observer's right eye.
An early three-dimensional projection system, known as a stereoscope, was popular in the 1850's. It used a system of lenses to display a separate image onto each eye of an observer. Since then, many methods of three-dimensional display have been proposed. Whatever the display method used, however, the quality of the three-dimensional image can only be as good as the two-dimensional images that are used to form it.
Two processes take place when the human eyes focus on an image. Firstly, the shape of the lens in each eye changes so as to alter the focal length to focus on an object. This process is known as “accommodation”. The second process is that the angle between the axis of the two eye changes to ensure that the object is focused simultaneously on the fovea of each eye (the fovea is the most sensitive part of the retina of the eye). This process is known as “convergence”.
FIG. 1A is a schematic illustration of a person looking at a distant object 3. The accommodation process will ensure that the lens in the left and right eyes 1L, 1R of the observer each change shape so as to focus on the object (assuming that the observer has normal vision). The convergence process will ensure that the point of convergence of the optical axis 2L of the left eye 1L with the optical axis 2R of the right eye 1R is coincident with the plane 4 containing the object 3.
FIG. 1B illustrates an observer looking at a closer object 5. The accommodation process ensures that the lens of each eye 1L, 1R changes shape so as to focus on the new image plane 6. The convergence process ensures that the point of convergence of the optic axes 2L, 2R of the two eyes alters so as to be coincident with the new image plane. The accommodation and convergence processes are not independent of one another, so that any mismatch between accommodation and convergence can lead to discomfort for the observer. This is a fundamental limitation of three-dimensional viewing.
The two different points of view of the two eyes of an observer produce images of objects on the retina that are different from one another. The difference depends on the distance of the object from the observer. The principle of a stereoscopic display is that the disparity between the image seen by the left eye and the image seen by the right eye is interpreted by the brain as indicating depth, and changes the eye convergence accordingly. However, as explained with reference to FIGS. 1A and 1B above, convergence and accommodation are not independent, and this has a limiting effect on a stereoscopic display.
FIG. 1C is a schematic illustration of a stereoscopic display that includes an image 3′ of the distant object 3 of FIG. 1A and also includes an image 5′ of the near object 5 of FIG. 1B. The stereoscopic image is being displayed on a display screen 7. The eyes of the observer will converge on a virtual object, such as the distant virtual object 3′ or the near virtual object 5′. As a consequence of this, and of the inter-dependence of the convergence and accommodation processes, the eyes will focus on the apparent depth of the distant virtual object 3′ or on the apparent depth of the near virtual object 5′. As a result, the plane of focus will not be coincident with the plane of the display screen, so that the virtual object will be out of focus if the apparent distance between the virtual object and the display screen is too great. Thus, virtual objects located too far out of or into the screen will cause the observer headaches and other discomfort.
A human can generally tolerate a certain amount of mis-match between accommodation and convergence without discomfort, and this allows a stereoscopic display to function within a limited depth either side of the display screen 7. The need to limit the depth of a virtual object behind or in front of a display screen places limitations on the parallax between the left eye image and the right eye image in the horizontal direction.
A further problem faced by a stereoscopic display is that the image presented to the left eye and the image presented to the right eye should not have objects that contain points that have been shifted vertically relative to other points in the scene—that is, the two images should not have “vertical disparity”.
Vertical disparity is illustrated in FIG. 2A to 2C. FIG. 2A shows a real world view that contains an object 8 that is near to an observer, an object 10 that is distant from an observer, and an object 9 that is intermediate in distance from the observer between the near object 8 and the far object 10.
FIG. 2B shows the left eye image 11L and the right eye image 11R, of the real world view, as captured by a stereoscopic image capture device or stereoscopic camera. (The term “camera” will be used herein for convenience of description, rather than “image capture device”. The term “camera” as used hereinbelow is intended to cover any device capable of capturing an image.)
FIG. 2C shows the result of superposing the left eye image 11L and the right image 11R of FIG. 2B. It will be noted that the left eye image of the near object 8 and the right eye image of the near object 8 are shifted horizontally and vertically relative to one another. The left eye image of the intermediate object 9 and the right eye image of the intermediate object 9 are also shifted relative to one another, but the horizontal shift and vertical shift are both considerably smaller than for the images of the near object 8.
Ideally, the two images presented to the eyes of an observer by a three-dimensional display system should not contain vertical disparity. Although the eyes can cope with a small amount of vertical disparity, this is only at the periphery of vision rather than at the fovea. The presence of such disparity in a converged imaging system such as the human eyes leads to keystone distortion, and this is corrected in the brain. In a stereoscopic display, therefore, no vertical disparity should exist so that the brain can correct the images on the retina properly. The design tolerance of a stereoscopic imaging system to the vertical disparity is small, and good images will not be obtained if there is significant vertical disparity present.
In the design of a stereoscopic camera, the camera arrangement and design are typically determined by the depth of the scene and displayed image (through the horizontal disparity) and the intended display method. However, the accuracy to which the features of the camera arrangement and design, such as the separation of the two cameras, the fields of view etc.) are specified depends on the maximum allowable vertical disparity in the scene which, for most scenes, is typically a very low level of vertical disparity. These requirements have placed significant limitations on the design, construction and use of stereoscopic cameras and thus make building a high-quality stereoscopic camera a difficult task. The horizontal and vertical parallax of the left-eye image and the right-eye image depends on many factors, such as the separation of the two cameras, the zoom and field view, the convergence of the optic axis of the two cameras, the display method etc. All these factors must be controlled in order to keep horizontal and vertical parallax between the two photographs within limits that will enable comfortable viewing of the resultant three-dimensional image.
Ideally there should be no vertical disparity between the left-eye image and the right-eye image. It is difficult to determine the accuracy to which this limit can be expressed, since this depends on what an average person would deem to be an acceptable level of vertical disparity. There have been numerous studies on this subject, but these have not provided any agreement as to what degree of vertical disparity is acceptable. In the case of an ideal digital display system the requirement that vertical disparity should be zero can be interpreted as meaning that the vertical disparity should be less than one pixel error. Errors in horizontal disparity that are greater than a single pixel manifest themselves as distortion of depth but, since depth distortion is present in stereoscopic images in any case, such errors do not cause great discomfort (as long as the horizontal disparity error is not too large). Thus, vertical disparity determines the accuracy with which the camera alignment and positioning must be specified in a three-dimensional camera system.
Recent developments in digital photography using digital cameras or scanners has made it possible to use computer software to correct a stereoscopic image pair that was obtained using a mis-aligned camera system, in which the positions of the cameras do not properly reflect the positions of the human eyes. This software makes it possible to use images obtained using a mis-aligned camera system, and so reduces the problem associated with physically aligning a camera system. However, this software is still at an early stage and currently available fully automated point matching correction software cannot properly correct for mis-aligned cameras. The software is either not sufficiently accurate to correct unaligned images properly, or it requires processing power and/or time that is simply not available on a small digital camera. Furthermore, scenes that contain a repeating pattern, (for example, a brick wall) can cause problems with the software, since there are many similar points on the two images and matching a point in one image with a corresponding point in the other image becomes difficult. A further disadvantage is that the software does not put information into an image, but it only corrects the orientation of the images—and it does this at the expense of image quality and possibly resolution (when cropping is involved).
The quality of such computer software is likely to improve with time. However, it will always be the case that the better the camera system is aligned, the shorter processing time will be required since it would be easier for the software to predict a matching point in the other image of an image pair. Good alignment of the camera system also helps where there are numerous similar areas in an image. Furthermore, the reduction in image quality and resolution are reduced if the cameras are correctly aligned.
The fundamental principle of a stereoscopic camera is the ability to acquire two separate images, one corresponding to the left eye image and the other corresponding to the right eye image. Many possible stereoscopic cameras have been proposed, but fundamentally they can be categorised into four different types, namely:    1. Single lens, single sensor system;    2. Multi-lens, single sensor system;    3. Single lens, multi sensor system; and    4. Multi-lens, multi sensor system.
FIGS. 3A(1) and 3A(2) show examples of a single lens, single sensor stereoscopic camera. FIG. 3A(1) shows a simple example of a stereoscopic camera system that comprises a camera 12 mounted for translation along a support such as an optical rail 13. The camera 12 may be moved between a first position in which it obtains an image for one eye and a second position in which it obtains an image for the other eye. The translational distance (d) between the two camera positions is approximately equal to the separation of the eyes of a human being. This distance (d) is also referred to as the “inter axial separation” of the two camera positions, since it is equal to the distance between the optic axis of the camera in its first position and the optic axis of the camera in its second position.
FIG. 3A(2) shows a more sophisticated stereoscopic camera system of the single lens, single sensor type. This stereoscopic camera 14 has a sensor 15 for capturing an image, and this sensor may be, for example, a CCD sensor. The camera 14 further has a lens 16 for focusing incoming light onto the sensor 15. A shutter 16 is disposed between the lens 16 and the sensor 15. The shutter has two independently controllable areas 17A, 17B, each of which blanks out approximately one half of the area of the lens 16. The area 17A of the shutter transmits, when open, light that would be perceived by the left eye and the area 17R transmits, when open, light that would be received by the right eye. FIG. 3(a) (2) shows the shutter 17 with the left eye area 17L in its open state and with the right eye area 17R in its closed state. The sensor 15 is therefore recording a left eye image. Once the left eye image has been recorded, the left eye area 17L of the shutter is closed, the right area 17R of the shutter is opened, and the sensor then records the right eye image. The left eye image and the right eye image together form a stereoscopic image pair.
The shutter 17 may conveniently be embodied as a liquid crystal display device (LCD), in which the left and right eye areas of the shutter 17L, 17R may be put in the “transmit” or “block” state by application of suitable voltage across the relevant part of the liquid crystal layer.
In the stereoscopic camera system of FIG. 3A(2), the inter axial—separation corresponds to the distance between the centre of the left eye area 17L of the shutter 17 and the centre of the right eye area 17R of the shutter 17.
As is clear from the above description, the two images that form a stereoscopic image pair are recorded time-sequentially in a single lens, single sensor stereoscopic camera system. A single lens, single sensor system fundamentally cannot therefore be used to obtain an error-free still stereoscopic image from a moving subject. Such a system does, however, have a high tolerance to mis-alignment of the camera.
FIG. 3B illustrates an example of a multi-lens single sensor stereoscopic camera system. The stereoscopic camera system of FIG. 3B has two lenses 16L, 16R for focusing incoming light on to a sensor 15 such as, for example, a CC sensor. The lenses are spatially separated in the lateral direction, so that one lens 16L receives light forming the left eye image and the other lens 16R receives light forming the right eye image. Light passing through a lens 16L, 16R is reflected by a mirror 19L, 19R onto a control mirror 20. The control mirror is switchable between a first position, shown in full in FIG. 3B, in which it completes the optical path from the left eye lens 16L to the sensor 15 (but blocks the optical path from the right eye lens 16R to the sensor), and a second position, shown in broken lines in FIG. 3B, in which it completes the optical path from the right eye lens 16R to the sensor 15 (and blocks the optical path from the left eye lens 16L to the sensor). Depending on the orientation of the mirror 20, therefore, the sensor records either a left eye image or a right eye image. The mirror 20 may oscillate backwards and forwards between its first orientation and its second orientation, or it may rotate continuously with pauses in the first and second positions.
A multi lens, single sensor stereoscopic camera of the type illustrated in FIG. 3B may be used to obtain a video stereoscopic image. Since the images forming a stereoscopic image pair are again recorded in a time-sequential manner it cannot obtain a still stereoscopic image from a moving subject.
FIG. 3C shows an example of a single lens, multi sensor stereoscopic camera. Incoming light is directed by a lens 16 onto two sensors. One sensor 15L obtains a left-eye image and the other sensor 15R obtains a right eye image. (The two sensors 15L and 15R may be embodied as two independently readable areas of a single sensor.) An optical system, formed in this example of mirrors 22L, 23L or 22R, 23R ensures that incoming light is directed onto the lens 16 in such a way that light forming the left eye image is directed onto the left sensor 15L, and that light forming the right eye image is directed onto the sensor 15R.
In a single lens, multi sensor system it is possible to obtain a left eye image and a right eye image simultaneously, and such a stereoscopic camera may therefore be used with a moving subject. A single lens, multi sensor stereoscopic camera also has the advantage that it can be applied to an existing camera, by fitting an appropriate incoming optical system. Such a system, however, has the disadvantage that it has a low tolerance to lens aberration.
FIG. 3D shows an example of a multi lens, multi sensor stereoscopic camera 24. This essentially comprises two conventional cameras 25L, 25R arranged side by side, and so a stereoscopic camera of this type is generally referred to as “stereoscopic camera pair”. One camera 25L captures the left eye image, and the other camera system 25R obtains a right eye image. Each camera contains a sensor 15L, 15R and an optical system for focusing light on the sensor (the optical system is represented in FIG. 3D by a lens 16L 16R).
A multi lens, multi sensor stereoscopic camera can record a left eye image and the corresponding right eye image simultaneously, and so can be used to obtain either a video image or a still image. It has a disadvantage that it has a low tolerance to misalignment of the two cameras relative to one another.
The present invention is directed to aligning a stereoscopic camera that has a low tolerance to misalignment, and is particularly applicable to a multi lens, multi sensor system as shown in FIG. 3D. It may also be applied to a multi lens, single sensor system of the type shown in FIG. 3B.
An example of the alignment accuracy required in a multi lens, multi sensor system of the type shown in FIG. 3D will now be made. If it is assumed that each camera 25L 25R in the camera system 24 has a CCD 15L, 15R that has 1280×960 pixels (giving a total of 1.3 million pixels, SXGA), in ⅔″ format (giving a pixel size of approximately 7 μm square) and with an 8 mm focal length lens, then the angular field sub tended by one pixel is about 0.9 mrad or 3 arcmin (1 arcmin equals ( 1/60°). In order for vertical disparity between a left eye image and a right eye image to be less than one pixel requires that the angular mis-match between the optic axis of one camera system and the optic axis of the other camera system must be less than 0.9 mrad in each plane. FIG. 4 illustrates a multi lens, multi sensor system of the type shown in FIG. 3D in which the optic axis of one camera 25R is misaligned relative to the optic axis of the other camera 25L. The above calculation indicates that the angular misalignment must be less than 0.9 mrad if the misalignment is not to affect the quality of a stereoscopic image pair captured by the camera. Misalignment can also occur as a result of rotation out of the plane of the paper in FIG. 4, and this mis-alignment must also be less than 0.9 mrad in order not to affect the image quality.
It should be noted that a higher resolution system with a longer focal length lens would require alignment to a greater accuracy than the above example.
The other source of mis-alignment in a multi sensor, multi lens stereoscopic camera system is translational errors—where the inter-axial separation of the two cameras is incorrect. The effect of such translational errors is shown in FIG. 5. FIG. 5 assumes that, if the two cameras 25L, 25R of a stereoscopic camera pair are positioned correctly as shown in full lines, the image of an object captured by the sensor 15R of the right camera 25R is centered on the sensor 15R. If the right camera 25R is not in its correct position, as shown in broken lines, the translational error causes the image to be off-centre on the sensor 15R of the right camera 25R. The distance by which the image is off-centre will depend on the translational error, and on the distance between the camera pair and the object. A translational error in the horizontal direction gives rise to horizontal disparity between the two images of an image pair, and a translational error in the vertical direction gives rise to vertical disparity between the two images of an image pair.
In addition to translational or rotational errors, there may also be other factors which cause misalignment between the optical axis of the two camera systems. Inaccuracies in the lenses of the camera systems, for example, may cause mis-alignment in the optical axis, and also cause the axis to shift with zoom, focus, aperture adjustment etc. Furthermore, tolerances in the manufacturing process mean that the two lenses 25L, 25R of the left and right camera systems are unlikely to be exactly identical to one another. As a result, production of a commercially successful stereoscopic camera pair is very difficult, owing to the difficulty in aligning and matching the two camera systems.
The difficulties in producing a stereoscopic camera pair can clearly be avoided by the use of a single sensor stereoscopic camera of the type shown in FIG. 3A or 3B, but these systems cannot take still photographs of moving subjects since the left-eye and right-views would be taken at different times. Single sensor systems are suitable for photograph so fast ill-subject, but many people do not want to be limited to a camera system that can not take photographs of moving subjects.
The use of a computer-based correction to compensate for the misalignment between the two cameras of a stereoscopic camera pair has been proposed, but no successful algorithm has been found. No successful algorithm has yet been found that can correct for lack of synchronisation between the two cameras of a stereoscopic camera pair which does not need knowledge about the scene photographed.
Stereoscopic camera systems based on holographic systems have been proposed. While overcoming some disadvantages of existing stereoscopic camera systems, holographic systems introduce their own problems (a coherent light source is required, recording a full colour image is difficult, etc.).
Various methods have been proposed for checking the alignment of the two cameras in a stereoscopic camera pair. One approach is to use a calibrated alignment chart of the type shown schematically in FIG. 6A(1). A stereoscopic camera pair 24 is calibrated by acquiring images of the calibration chart 26 using the left camera 25L and the right camera 25R, and comparing the image acquired by the left camera 25L with the image obtained by the right camera 25R. This process is shown schematically in FIG. 6A(2). The two images are analysed, and the results of the analysis are used to correct the relative alignment of the two cameras 25L, 25R of the stereoscopic camera pair 24. The images may be analysed either by hand, or by using a computer point matching algorithm.
This prior art technique has a number of disadvantages. The principal disadvantage is that the use of a calibration chart at a finite distance from a stereoscopic camera pair does not enable the alignment errors to be decomposed into translational errors and rotational errors. This is illustrated in FIG. 6B. The camera head M shown in broken lines in FIG. 6B is the result of translating the right camera head 25R from its correct position (shown in full lines as “head A”) and also rotating the right camera head relative to the left camera head (shown as “head C”). As is shown schematically in FIG. 6B, it is possible that the combination of a translational error and a rotational error will place the image of one calibration point of the calibration chart 26 on the same point of the sensor in both the correctly aligned camera head A and the mis-aligned camera head M. As a result, a calibration chart will indicate, incorrectly, that the camera head A was correctly aligned with the camera head C. This happens because a rotational error about the separation axis (pitch) produces the same disparity error on the sensor as a translation (along an axis in the plane of the paper in FIG. 6B). It is not possible to separate translational errors and rotational errors unless the calibration chart is positioned far away from the stereoscopic camera system that is being aligned, and this would require the use of a large, accurately calibrated chart or a chart that has a three-dimensional nature to it (i.e., not a flat chart)—which would be both difficult to handle and expensive.
The use of a calibration chart also does not separate errors introduced by the lenses from errors introduced by the alignment of the sensors.
Another prior art approach to aligning the two cameras of a stereoscopic camera pair is computer-based analysis of calibrated charts, or non-calibrated scenes to determine parameters indicative of the mis-alignment of the two cameras. This process is shown schematically in FIG. 7A and FIG. 7B. FIG. 7A shows a typical scene used in this method. Each of the left and right cameras 25L, 25R acquires an image of this scene as shown schematically in FIG. 7B. The image from the left camera 25L and the image from the right camera 25R are analysed, and alignment parameters required to correct one of the images to match the other are found. This analysis may again be carried out by hand, or using a computer point matching algorithm. Manual analysis of the images can be slow, and is also tedious for the operator. Computer-based point matching can be faster, but it may well not reliably achieve sub-pixel accuracy if a non-calibrated scene was used. A further disadvantage is that this method cannot separate errors introduced by lens alignment/inaccuracy from sensor alignment errors. As a result, corrections required for an object in the scene at one apparent depth from the camera may require different correction parameters from an object at a different apparent distance from the camera.
Computer post-processing of images has also been suggested. However, post-processing of images of known errors suffers from similar problems to those outlined above. In addition, interpolation of images will degrade the quality of the final image.
The use of photo-geometric methods using lasers to measure the parameters of a conventional single camera has been proposed by T. A. Clarke et al., in “The principal point and CCD cameras” Photogrammetric Record Vol 16, No. 92 pp 293–312 (1998), and this method is illustrated schematically in FIGS. 8A and 8B. Initially, as shown in FIG. 8A a laser beam is directed towards a camera from which the lens is absent. The laser beam is aligned so as to be coincident with the optic axis of the camera, and to be incident on the centre of the sensor 15.
Once the laser beam has been correctly aligned, the lens 16 is inserted into the camera as shown in FIG. 8B. The position that the laser beam now makes on the sensor of the camera determines the “auto-collimation” position of the lens systems, and this is a measure of the inaccuracy in the lens 16. It is possible to derive a correction parameter for correcting photographs acquired by the camera.
Japanese Patent Application No. 9-312 808 (Sony Corporation) discloses a system for aligning a lens to a CCD sensor. The method involves placing marks on the CCD sensor, and aligning the lens relative to the marks to ensure correct alignment of the lens relative to the CCD. Although it might be possible to use such a method to align two CCDs relative to an external datum, in order to achieve the desired accuracy (less than one pixel to prevent vertical disparity) it would be necessary to position the marks on the CCDs to within an error of less than one pixel size. As a pixel typically has a dimension of less than 10 μm, positioning the marks to the required accuracy would be extremely difficult to do.
U.S. Pat. No. 5,877,854 discloses an alignment system for an optical device using two sensors and two light sources. Laser beams are projected downwards onto line CCD sensors. This apparatus can achieve the required alignment accuracy with regard to rotation about one axis, but cannot provide accurate alignment with regard to alignment about three orthogonal axes.
M. Aggerwal et al disclose, in “Camera Centre Estimation”, Proc. 15th International Conference on Pattern Recognition, Vol. 1 pp 876–80 (2000) a method of determining the optical centre of a camera system. The method uses two calibration charts, and a computational post-processing method. The quoted accuracy is around 3.6 pixels, which is too large an error for this method to be suitable for correcting the alignment of two cameras in a stereoscopic camera pair.
EP-A-0 506 039 discloses an apparatus for measuring the position of the edge of a cutting tool. The apparatus has a “reference plate” that is positioned near the edge of the cutting tool, so that a slit is defined between the reference plate and the edge of the cutting tool. The apparatus also has a laser for generating a light beam that is directed at the slit defined between the reference plate and the edge of the cutting tool. The slit diffracts the light beam, and the resultant diffraction pattern provides information about the width of the slit and hence about the position of the cutting tool relative to the reference plate.
U.S. Pat. No. 5,073,918 discloses a method of X-ray crystal diffraction used for determining the orientation of the crystal axes of a silicon wafer. A beam of X-rays is directed at a silicon wafer which is mounted on a goniometer in the path of the X-rays, and the resultant X-ray diffraction pattern is detected by a photosensor array.
U.S. Pat. No. 3,787,117 discloses an apparatus for inspecting a work piece. A workpiece having a repeat pattern, such as a photo mask, is positioned in the path of light from a source. A mask having two apertures is disposed behind the workpiece, and the apertures of the mask give rise to diffraction patterns. The diffracted images then pass through a grating which is asymmetrically positioned with respect to the optical axis of the apparatus. The resultant image is displayed on a screen, and provides information about non-periodic errors in the workpiece.
U.S. Pat. No. 3,656,838 discloses a filter for use in an optical character identification system. A plate displaying a character is placed in a light beam, and the light output from the plate passes through a two-dimensional diffraction grating. The diffracted image then passes through a filter. The final image provides information about the particular character displayed on the plate.